This invention relates to oil recovery using a modified heteropolysaccharide stabilized against pyruvate loss on thermal treatment. More particularly, the heteropolysaccharide used for oil recovery is prepared using a thermal treatment carried out in the presence of a buffered saline heteropolysaccharide solution.
U.S. Pat. No. 4,182,860 describes a process for preparing a modified heteropolysaccharide derived from bacteria of the genus Xanthomonas using a saline heat treatment which results in improved filterability of the so-treated heteropolysaccharide. The saline heat treatment, however, results in a .about.20% loss of pyruvate groups. Sanford et al., Extracellular Microbial Polysaccharides, Symposium presented at 172nd Meeting of the American Chemical Society, ACS Symposium Series No. 45, Sandford and Laskin ed., pages 192-210 describe Xanthan heteropolysaccharides of differing pyruvic acid content. This paper shows that pyruvate content affects polysaccharide properties such as viscosity. Viscosity is in turn influenced by such factors as temperature, salt and pH. U.S. Pat. No. 3,801,502 discloses a method for producing waterflood bacterial viscosifiers wherein an alcohol, phenol, ketone or nonionic surfactant is added to fermentation effluent and the resulting mixture heated at temperatures between 50.degree. C. and its boiling point until the desired viscosity is attained. Water or brine may be used as a reaction medium. U.S. Pat. No. 3,773,752 teaches a process for recovering polysaccharides from a heteropolysaccharide solution which comprises diluting the heteropolysaccharide solution with a salt solution until a desired viscosity is attained. Coagulation can be achieved by heating to a temperature below the boiling point of the mixture.
It is well known to add buffers to fermentation broth to optimize conditions for the fermentation process. In the case of bacteria of the Xanthomonas genus, a pH between about 6.0 to 7.5 is preferred. NA.sub.2 HPO.sub.4, K.sub.2 HPO.sub.4, NaH.sub.2 PO.sub.4 and KH.sub.2 PO.sub.4 are buffers typically added for this purpose. For reference see U.S. Pat. Nos. 4,119,546 and 3,020,207.
Jeanes et al., J. Appl. Polymer Sci., 5, 519-526 (1961) describe the heat treatment of polysaccharide B-1459 wherein the polyscaccharide was dissolved in water containing 1% KCl at pH 6.5, heated at 90.degree. C. for 30 min., cooled and precipitated. U.S. Pat. No. 3,591,578 teaches that polysaccharide degradation in the fermentation broth can be avoided by heating the broth prior to precipitation at a temperature of from 80.degree. C. to 130.degree. C. for 10-120 minutes at a pH of from 6.3 to 6.9. U.S. Pat. No. 3,355,447 relates to a method of preparing a Xanthomonas colloid of improved stability in which the fermentation beer is adjusted to pH 7.0 to 9.0, heated at temperatures of from 150.degree. to 170.degree. F. for at least 20 minutes.
Patton, SPE Paper No. 4670, 48th Annual Meeting of the SPE of AIME, Las Legas, 1973, reports on a xanthan gum which is modified by adding a base such as NaOH or KOH to the polymer to raise the pH to about 12, then heating to about 185.degree. F. for a period of 5-20 minutes. U.S. Pat. No. 4,096,073 discloses a method for increasing the viscosity of fermenter suspensions containing Gram-negative microbial cells comprising contacting the suspension with a base to raise the pH to a value between 9 and 12, and then aging at 50.degree.-212.degree. F. for at least 5 minutes. Holzwarth et al., Carbohydrate Research, 76, 277-280 (1979) demonstrate that pyruvate of acetyl groups can be selectively removed from xanthan. Pyruvate groups were removed by heating a dilute saline solution of xanthan at pH 3 for 2 hours at 95.degree. C.
It would be desirable to have a heteropolysaccharide which has been modified by saline heat treatment and which has a pyruvate content substantially unchanged from that of the starting heteropolysaccharide. An unchanged pyruvate content is desirable to obtain a viscosifier with optimal viscosifying properties.